Bug 779979 - Be more explicit about why mozharness can't accept a config file with wrong extension r=aki

MozReview-Commit-ID: 29pQRxQzbSK

--HG--
extra : rebase_source : 6c09d80edaba168e7371201678f33f3b24ecb83e

testing/mozharness/mozharness/base/config.py

@@ -165,7 +165,7 @@ def parse_config_file(file_name, quiet=False, search_path=None,
         config = dict(json_config)
         fh.close()
     else:
-        raise RuntimeError("Unknown config file type %s!" % file_name)
+        raise RuntimeError("Unknown config file type %s! (config files must end in .json or .py)" % file_name)
     # TODO return file_path
     return config